Keypad brightness on a handheld electronic device may be adjusted for different operating environments in order to enhance the visibility of key indicia (e.g. alphabetic characters, numbers). Conventional backlit keypads or keyboards, such as Logitech Illuminated Keyboard part number 920-000914 or MacBook Air part number MC233LL/A, are illuminated with a monochrome light source. A backlight controller varies the intensity of the monochrome light source based on measurements of ambient light intensity. Alternatively, if no light sensor is provided it is customary to illuminate the backlight whenever the device is powered on, to ensure that the keypad is readable in the dark.
Typically, the keys of a conventional backlit keypad are opaque, for example black with key indicia or characters thereon that are transmissive in order to allow light to pass through for the purpose of backlighting in dark environments and that are white or some other colour that creates suitable contrast for legibility. The backlight controller activates the keypad backlight when the keypad is exposed to dim or moderately-intense ambient light, and deactivates the keypad backlight when the keyboard is exposed to bright ambient light (e.g. in outdoor or sunlight conditions).
Cathey (U.S. Pat. No. 7,129,930) describes a backlit keyboard in which each key includes dark-coloured key indicia printed on a transparent light-coloured key face. Faraqe (U.S. Pat. No. 6,797,902) describes a keyboard in which each key is coupled to a large lamp that illuminates the transparent key face, and a small lamp that illuminates the dark-coloured key indicia. Knox (U.S. Pat. No. 6,950,087) describes a backlit keyboard in which the keys are illuminated by a variable-position prism that allows the user to manually adjust the colour of the backlight. Stokic (U.S. 2006/0061542) describes a keyboard that is coupled to a display controller. The display controller transmits key indicia to the keys, and adjusts the intensity of the key indicia, and the contrast between the background and the key indicia, based on the ambient light level. Kreek (U.S. 2007/0097065) sets forth a method for automatically adjusting screen and keypad brightness of non-contrast inversion keypads to improve legibility of the keypad on a mobile communication device, whereby in the event the ambient light level exceeds 50 Lux for 30 seconds the keypad backlight switches from on to off, and in the event the ambient light level falls below 16 Lux the keypad backlight switches from off to on. Sellers (U.S. 2007/0068784) sets forth a keyboard with lightpipes directing light to the underside of the keys.
A problem of readability of characters on a keypad or keyboard (or other input device, such as an instrument panel) occurs for a certain range of ambient lighting conditions where contrast inversion occurs (i.e. when a character changes from being a dark element in high ambient light conditions to being a light element in low ambient light conditions). This range of ambient lighting conditions occurs when there is ‘dim’ lighting (i.e. between daylight and darkness) such that the contrast ratio of the characters on the keypad decreases below a value at which they are no longer legible.